This invention relates to xerographic printers or copiers and more particularly to co-binders for use in the charge layer of the electrophotographic photoreceptor portion of the xerographic xe2x80x9cdrumxe2x80x9d or xe2x80x9cplatexe2x80x9d utilized in such printers or copiers.
Xerographic printers operate by passing the photoconductor drum surface close to a positively or negatively charged source, usually a wire or a charge roll to create a charge on the drum. The material to be copied is projected onto the photoreceptor through appropriate lenses and the charge on the drum is discharged from different portions of the photoconductor proportionately to the intensity of the light projected on different parts of the photoreceptor. The photoreceptor is then exposed to oppositely charged xe2x80x9ctonerxe2x80x9d powder or powders as needed to retain a coating of desired color or colors on the drum. The toner coated drum is then exposed to an oppositely charged copy material which is generally paper, plastic or fabric. The charged copy material attracts the toner onto its surface to form the desired image. On heating, the toner is permanently fixed on the copy material surface.
Typically, a photoreceptor coating generally includes a charge generation layer (CGL) and a charge transport layer (CTL) which overlay one of a xe2x80x9csubxe2x80x9d or xe2x80x9cbarrierxe2x80x9d layer coating the drum or plate surface itself. The CGL preferably is made up of pigments or dyes dispersed in a polymer binder but the pigments can also be dispersed in a liquid solvent. The binder improves the dispersion stability and improves adhesion of the CGL to the plate and sub or barrier layers on the plate.
Typical pigments and dyes utilized in photoreceptors include one or more phthalocyanines, squaraines, azodyes, perylenes. Typical polymer binders are the polyvinyl butyrals, phenoxy resins, epoxy resins, polycarbonates and polyacrylates. The binders are essentially inert with respect to the desired electrophotographic properties of the CGL. If not properly formulated, however, the pigments and/or binders can affect the sensitivity of the photoreceptor. Similarly, the CTL also contain polymeric binders. Typically, the binders can typically include one or more of polycarbonates, polyesters, copolymers of polymers having reactive ester and carbonate groups, phenoxy resins, epoxy oxy resins, and silicones. The CTL is typically about 5 to about 40 microns in thickness. The preferred dual layer negative charging photoreceptors of this invention exhibit improved electrical stability, lower dark decay and form stable prints through the life of the photoreceptor. However, the photoreceptors can also be a part of a positive charge system when hole transport materials are substituted, at least in part for the preferred electron transport material.
Exemplary references include:
U.S. Pat. No. 6,001,523 issued to Kemmesat, Neely, Randolph and Srinivasan; U.S. Pat. No. 6,042,980 to Kierstein and Srinivasan; and U.S. Pat. No. 6,117,967 to Fuller, Yannus, Pai, Silvestri, Naran, Limberg and Renfer. These patents summarize and/or refer to a variety of photoconductor technology.
Aliphatic carbonate diols are co-binders in the electrophotographic portion of xerographic plates and drums utilized in xerographic copiers. The carbonate diols have the structural formula: 
where R is an aliphatic hydrocarbon containing about 3 to about 13 carbon atoms, m is about 1 to about 7 and n is about 3 to about 50. Preferably, R contains about 3 to about 13 carbon atoms; more preferably R contains about 4 to about 10 carbon atoms; and most preferably R contains about 4 to about 6 carbon atoms. Preferably, n is about 3 to about 30 and more preferably about 3 to about 15.
The aliphatic hydrocarbon moieties are preferably linear but can have a low molecular weight primary and secondary substitution, e.g., methyl and isopropyl.
The preferred polyaliphatic carbonate diols are:
poly(hexamethylene carbonate) diol (PHMC) having the formula:
HO [xe2x80x94CH2(CH2)4CH2OCO2xe2x80x94]nCH2(CH2)4CH2OH
where m is 1 hexamethylene unit and n is 3 to about 15, a poly(polytetrahydrofuran carbonate) diol having the formula:
H(OCH2CH2CH2CH2)xe2x80x94[OCO(OCH2CH2CH2CH2)m]nOH
where m is 3 to about 4 polytetrahydrofuran units and n is 3 to about 15 repeating units.
These co-binders have number average molecular weights ranging from about 200 to about 10,000, more preferably from about 200 to about 5000 and, most preferably from about 200 to about 2000. Preferably these polymeric CTL compounds range from flexible viscous semi-solids to substantially rigid solids at room temperature. More preferably, these materials are flexible viscous solids.
The preferred binders for CTL formation are polycarbonates, polyesters, polystyrenes, polyvinyl chloride, epoxy resins, phenoxy resins, polyvinylbutyral and vinyl chloride/vinyl acetate copolymers and mixtures thereof. The most preferred binder compositions are with polycarbonates, polyesters, polystyrenes, and mixtures thereof. The use of polycarbonates is discussed in depth in U.S. Pat. No. 6,001,523. These binders can be used with various charge transport compounds, such as aryl amines, benzidines, hydrazones, stilbenes and mixtures thereof. In addition to the polymer binders in the charge transport layer, additives such as polytetrafluoroethylene, and polysiloxanes can be used, so as to help improve the wear properties of the photoconductor drum.
The amount of co-binders utilized depends on the effects of specific binders and other constituents of the CTL as well as the costs of the specific co-binder chosen for use in a pre-selected CTL composition. Co-binder concentrations are subjects of cost benefit analysis. In some situations, co-binder concentrations greater than about 10% by weight may be useful, but concentrations up to about 7.5% are currently preferred and concentrations of about 5% provide practical physical and price benefits.
In the following examples, the co-binder material is formulated in a solution or dispersion with a benzidine, e.g., N,Nxe2x80x2-di(3-methylphenyl)-4,4xe2x80x2-diphenyl benzidine (TPD) or N,N-diethylaminobenzaldehyde- 1,1-diphenylhydrazone (DEH) transport material by mechanically stirring the charge transport layer ingredients preferably in a suitable solvent or solvent mixture at temperatures ranging from 25xc2x0 C. to about 50xc2x0 C. Solvents typically used in preparing charge transport solutions or dispersions include but are not limited to tetrahydrofuran, dioxane, dioxolane, halogenated hydrocarbons, ketones, esters and mixtures thereof. A minor amount of surfactant is added, where needed, to achieve the proper degree of dispersion or solution. The charge transport binder blends preferably utilize one or both of polycarbonate-A and polycarbonate Z. Polycarbonate-A (PC-A) utilized in the examples set out below are marketed under the tradename Makrolon was obtained from the Coating and Colorants Division of The Bayer Corporation, Pittsburgh, Pa. USA. Polycarbonate-Z (PC-Z, lupilon-400Z) was obtained from Mitsubishi Engineering Plastics of New York, USA. The resulting PHMC diol co-binder was used at about 5% by weight of the binder in the charge transport layer.
This formulation was coated on a 45% type IV oxotitanium phthalocyanine pigment dispersed in a polyvinyl butyral/epoxy resin blend. Polyvinylbutyral (S-Lec-B BX-55Z grade) was obtained from Sekisui Chemical, New York. Epoxy resin was obtained from Shell Chemical.
Polyhexamethylene carbonate diol (PHMC), marketed by Aldrich Chemical Company, Wisconsin, USA was formulated as a co-binder in a TPD-based charge transport layer. The blend of binder and co-binder corresponds to a mixture with polycarbonates (PC), specifically polycarbonate-A and polycarbonate Z. One blend was with 100% Polycarbonate A and the other contained 25% Polycarbonate Z. About 5% co-binder was added to each of the blends. The formulations were coated on a 45% type IV oxotitanium phthalocyanines pigment dispersed in a polyvinyl butyral/epoxy resin blend. The coated drums were used as one of Ultraviolet (UV) radiation cured and non-UV cured and evaluated for life in a Lexmark Optra S 2450 printer.
In the various tests, the control drums were coated with blends containing no PHMC diol, but containing 2% TOSPEARL (Tospearl-120) silicone microspheres. The silicone microspheres were manufactured by GE Silicones of New York, USA. The silicone microspheres have been previously shown to exhibit superior electrical and print performance with respect to wear, coating quality and print quality (U.S. Pat. No. 5,994,014).
The formulation and results are presented below: